A biphase signal can be conceptualized as comprising a series of cells each providing clock and data information. One type of biphase signal is the biphase-L signal wherein each cell has a mid-cell level transition providing both clock and data information. The time of occurrence of the transition provides the clocking information, and the direction of the level transition supplies the data information. For example, a mark is encoded as a positive going transition and a space as a negative going transition. It will be appreciated that a level transition will occur at a cell boundary whenever adjacent cells are encoded with the same data information. For example, two successive cells encoded with a mark signal will each have positive going mid-cell level transitions separated by a negative going cell boundary transition. The decoding circuitry for a biphase-L signal typically ignores any cell boundary transitions and responds only to mid-cell transitions. Prior biphase signal receivers are described in U.S. Pat. No. 4,567,604 Issued Jan. 28, 1986 to
B. H. Jacksier and entitled "Biphase Signal Receiver" and U.S. Pat. No. 4,260,952, issued Apr. 7, 1981 to J. W. Thomas and entitled "Circuit For Decoding a Diphase Signal", both having a common assignee with this application.
Frequently, the biphase signal is distorted requiring the sampling of many segments of a bit cell to accurately decode the signal. The amount of signal distortion which a receiver can accommodate is related to the sampling rate, the greater the number of samples the more signal distortion the receiver will accommodate. Thus, the ability of a biphase receiver to accommodate distortion is directly related to the frequency of the internal clock controlling the signal sampling. Prior asynchronous biphase signal receivers have often incorporated a high frequency clock with a rate as high as sixteen times the frequency of the embedded clock of the biphase-L signal. Such high frequency signals require circuit components having very high switching speeds thereby significantly increasing the cost of the receiver.